It is known to use X-rays for transmission imaging in baggage scanning facilities at airports and the like.
For many reasons low energy X-rays have been employed hitherto. Not least of the reasons is the difficulty and cost of providing adequate screening for the users and the general public, which escalate as X-ray energies employed are increased. Other reasons have to do with the absorption characteristics of materials to X-rays and a greater variation is observed at low energies (for example up to 200 KeV) than is exhibited at higher energies (e.g. above 1 MeV), thereby making low energy X-ray imaging more sensitive to thickness differences.
Unfortunately low energy X-rays cannot penetrate as well as high energy X-rays and whereas the former can be used to penetrate the walls and contents of briefcases and handbags and ordinary luggage, they are of little use when containers are steel lined or made of steel as for example are shipping containers, goods waggons and the like. For such applications only high energy X-rays (in excess of 1 MeV) can be used. This has required massive concrete structures to provide the necessary shielding and because of the relatively small variation in absorption as between one material and another at such elevated energy levels, such facilities have been restricted to X-ray imaging techniques involving the production of a visual display (as on a VDU) of the interior of the object under test, which has then had to be monitored by personnel to determine if the contents are deemed to be hazardous or illegal.
Using low energy X-rays, baggage interrogation can also include determination of the mean atomic number of the materials within the container scanned. This enables a mean atomic number profile to be generated for each item. However this technique has not hitherto been possible when high energy X-rays have to be employed (to penetrate the items under inspection) since the absorption variation at these higher energies was deemed not to be sufficiently great as to admit ready discrimination between one material and another.
To this end it is an object of the present invention to provide a method and apparatus for X-ray inspection using high energy X-rays which permits discrimination on the basis of atomic number between materials exposed to the X-rays.
It is also an object of the invention to provide a method and apparatus as aforesaid by which containers such as steel shipping containers as used for road, rail and maritime freight can be X-rayed and a mean atomic number profile generated of their contents for analysis using conventional image analysis techniques, to identify the presence of particular substances or combinations of substances within the container, whereby an alarm signal can be generated if one or more criteria is satisfied, so as for example to prevent the loading or subsequent transit of a container so identified.